Abstract:

Apparatus to control a fluid flow are disclosed. An example fluid flow
control apparatus described herein includes a signal stage comprising a
signal stage relay having a supply plug being operatively connected to a
valve seat at a first end and an exhaust seat at a second end and a seal
operatively coupled to the supply plug such that the seal provides a
feedback area to apply a fluid pressure feedback force to the exhaust
seat.

Claims:

1. A fluid flow control apparatus comprising:a signal stage comprising a
signal stage relay having a supply plug operatively associated with a
valve seat at a first end and an exhaust seat at a second end; anda seal
operatively coupled to the supply plug such that the seal provides a
feedback area to apply a fluid pressure feedback force to the exhaust
seat.

2. The apparatus of claim 1, further comprising means for urging a seat
load across the supply plug toward either the valve seat or the exhaust
seat.

3. The apparatus of claim 1, wherein a spring is operatively coupled to
the supply plug to overcome a frictional force created by the seal.

4. The apparatus of claim 3, wherein the exhaust seat includes an input
post to contact an input linkage such that a bias force of the spring is
to maintain contact between the input linkage and the input post to
substantially reduce a dead band between input linkage motion and exhaust
seat motion.

5. The apparatus of claim 1, wherein the fluid pressure feedback force is
proportional to the signal stage output pressure.

6. The apparatus of claim 1, further comprising a signal stage relay
housing such that the signal stage relay housing and the seal define a
signal stage module that provides a predetermined feedback area adapted
to operate with a predetermined linkage force.

8. The apparatus of claim 7, wherein a first end of the supply plug is
substantially in contact with the valve seat and a second end of the
supply plug is substantially in contact with the exhaust seat at a
quiescent point in the throttling mode.

9. A dual-stage fluid flow control apparatus, comprising:a signal stage
having a proportional output, the signal stage comprising a signal stage
relay including a supply plug having a first end adjacent a valve seat
and a second end adjacent an exhaust seat, a signal stage input post
adapted to couple the signal stage to a control device and means for
urging a seat load across the supply plug toward either the valve seat or
the exhaust seat; andan amplifier stage comprising an amplifier stage
relay operatively connected to the signal stage via a signal passage, the
amplifier stage having a fluid supply responsive member adapted to move a
relay member to provide an amplified fluid supply output such that a
shift in the seat load across the valve seat and the exhaust seat
provides a predetermined engagement of either the valve seat to the first
end of the supply plug or the exhaust seat to the second end of the
supply plug to provide either a proportional or snap-acting and a direct
or reverse acting output of the amplifier stage relative to an input
signal at the signal stage input post.

10. The apparatus of claim 9, wherein the shift in seat load provides an
adjustment in either valve seat leakage or exhaust seat leakage during
quiescent operation of the signal stage to adjust a pressure balance
across the signal stage and the amplifier stage in proportion to a sensor
signal at the signal stage input post.

11. The apparatus of claim 9, wherein a fluid pressure in the signal
passage acts upon an inner surface of a signal stage o-ring to apply a
negative feedback force to provide the proportional output of the
amplifier stage.

12. The apparatus of claim 11, wherein a force equal to the product of the
pressure within the signal passage and an effective sealing area defined
by the inner surface of the signal stage o-ring is applied in opposition
to an input force on a signal stage input post.

13. The apparatus of claim 9, wherein a first stabilizing pressure
regulator provides a fluid supply to the signal stage and a second
stabilizing pressure regulator provides a fluid supply to the amplifier
stage.

15. The apparatus of claim 14, wherein the first end of the supply plug is
substantially in contact with the valve seat and the second end of the
supply plug is substantially in contact with the exhaust seat at a
quiescent point in the throttling mode.

16. A dual-stage, fluid flow control apparatus comprising:a signal stage
having a proportional output, the signal stage comprising a signal stage
relay including a supply port, a supply plug having a first end adjacent
a valve seat and a second end adjacent an exhaust seat, a signal stage
input post adapted to couple the signal stage to a control device and
means for urging a seat load across the supply plug toward either the
valve seat or the exhaust seat; andan amplifier stage comprising an
amplifier stage relay operatively connected to the signal stage via a
signal passage, the amplifier stage relay having a fluid supply
responsive member adapted to move a relay member to provide an amplified
fluid supply output, wherein a shift in the seat load across supply plug
of the signal stage closes the exhaust seat of the signal stage prior to
opening the valve seat of the signal stage to substantially eliminate a
transition bleed in the signal stage.

17. The apparatus of claim 16, wherein the first end of the supply plug is
substantially in contact with the valve seat and the second end of the
supply plug is substantially in contact with the exhaust seat at a
quiescent point in the throttling mode.

18. The apparatus of claim 16, wherein a seal is operatively coupled to
the supply plug such that the seal at least partially defines a feedback
area that yields a fluid pressure feedback force to the exhaust seat.

19. The apparatus of claim 18, wherein a spring is operatively coupled to
the supply plug to overcome a frictional force created by the seal.

20. The apparatus of claim 18, wherein the fluid pressure feedback force
is proportional to the signal stage output pressure.

21. The apparatus of claim 19, wherein the exhaust seat includes an input
post to contact an input linkage such that a bias force of the spring is
to maintain contact between the input linkage and the input post to
substantially eliminate a dead band between input linkage motion and
exhaust seat motion.

Description:

CROSS REFERENCE TO RELATED APPLICATION

[0001]This patent claims the benefit of U.S. Provisional Patent
Application Ser. No. 61/201,059, filed on Dec. 5, 2008, entitled
APPARATUS TO CONTROL FLUID FLOW, which is incorporated herein by
reference in its entirety.

FIELD OF THE DISCLOSURE

[0002]This disclosure relates generally to fluid flow control devices and,
more particularly, to apparatus to control fluid flow.

BACKGROUND

[0003]Industrial processing plants use control devices in a wide variety
of applications. For example, a level controller may be used to manage a
final control mechanism (i.e. valve and actuator assembly) to control the
level of a fluid in a storage tank. Many process plants use a compressed
gas, such as compressed air, as a power source to operate such control
devices. In certain hydrocarbon production facilities, compressed air is
generally not readily available to operate the control devices. Natural
gas is often used as the supply gas to operate these control devices.
However, many control devices may bleed natural gas to the atmosphere,
which is costly due to the value of the natural gas and the environmental
controls and regulations associated with such exhaust gases. Thus,
minimizing or eliminating the bleed of natural gas to the atmosphere by
the control devices is an important concern.

[0004]It is generally understood that typical level controllers used in
the hydrocarbon production industry may be single stage, low-bleed
pneumatic devices operated by natural gas. To minimize the consumption of
natural gas during operation, such level controllers are designed to
include a dead band to reduce amounts of bleed gas. However, such designs
generally have low operational sensitivity or gain resulting in large
vessel spans or oversized sensors.

[0005]It is also common to improve the gain of such single stage devices
by fashioning a dual-stage pneumatic control device to produce the
desired response characteristic with higher output sensitivity. The first
stage, often called the signal stage, converts a mechanical or fluid
pressure input signal to a pressure output. The signal stage has a low
volume flow rate and a low-pressure output that provides the response and
control characteristics for the desired process control application. A
second stage, often called the amplifier stage, provides high pneumatic
capacity and responds to the output of the signal stage to achieve the
desired response characteristics while providing a higher output flow
rate and/or pressure necessary to operate the final control mechanism.
Many of these devices do not provide control action proportional to an
input signal and/or suffer from excessive loss of supply gas, such as
natural gas, during operation.

[0006]FIG. 1 and FIG. 2 illustrate a known direct-acting, dual-stage
pneumatic control device 1 that includes a reverse-acting signal stage A
comprising a signal stage valve 110 coupled to a reverse-acting amplifier
stage B having an amplifier stage relay 10 (as explained in greater
detail below). In operation, an input signal (such as a motion or
displacement) from a mechanical device, such as a linkage connected to a
displacer in a fluid tank (not shown), may be applied to a valve stem tip
135 of the signal stage valve 110 to initiate a pneumatic control signal
to the amplifier stage relay 10. However, it should be appreciated by
those of ordinary skill in the art that the input signal might also be
derived from any number of well-known inputs including pressure signals
and other direct mechanical forces.

[0007]The amplifier stage relay 10 of the amplifier stage B is the
four-mode pneumatic relay disclosed in U.S. Pat. No. 4,974,625, which is
hereby incorporated by reference herein in its entirety. Those desiring
more detail should refer to U.S. Pat. No. 4,974,625. This relay provides
user selectable direct or reverse and proportional or snap-acting
operational modes. One of ordinary skill in the art appreciates that a
direct or reverse acting mode refers to the relationship of the output
signal with respect to an input signal such that, for example, direct
mode means the output signal increases with an increasing input signal.
Whereas a proportional or snap-acting mode refers to the response of the
output signal such that, for example, proportional means changes in the
output signal are substantially linear with respect to an input signal
change and snap-acting means changes in the output signal are bi-stable
and non-linear with respect to an input signal change.

[0008]Although the pneumatic relay disclosed in U.S. Pat. No. 4,974,625
may provide four modes, the dual-stage pneumatic control device 1
illustrated in FIG. 1 and FIG. 2 may disadvantageously utilize only two
modes of operation--direct and reverse/snap-acting modes. This is because
the dual-stage pneumatic control device 1 provides very little feedback
or proportioning force between the amplifier stage relay 10 and the
signal stage valve 110. That is, there is no specific mechanism to
feedback output pressure from a signal diaphragm 90 of the amplifier
stage relay 10 to offset the applied input force at the valve stem tip
135 of the signal stage valve 110.

[0009]In general, the amplifier stage relay 10 of the control device 1
includes a series of input and output ports that communicate with
respective chambers formed within the amplifier stage relay 10. By
selectively controlling the fluid communication between various input and
output ports through the user selectable switches, the single amplifier
stage relay 10 may provide the multiple operational modes previously
described to interface with various control elements.

[0010]Referring to FIG. 2, to accommodate the operational modes in the
amplifier stage relay 10, an input port 11 communicates with a chamber 15
and an output port 12. A pressure outlet 17 communicates with a chamber
16; an input port 13 communicates with a chamber 18, an output port 14
communicates with chamber 20 and the pressure outlet 17 may be connected
to a final control mechanism such as a valve and actuator assembly (not
shown).

[0011]FIG. 1 shows a cut-away illustration of the port switches of the
amplifier stage B of the control device 1 used to select the various
operational modes. First and second generally triangular-shaped port
switches 70 and 72 are pivotally mounted on the amplifier stage relay 10
by pins 71 and 73, respectively. The port switches 70 and 72 are
sectioned to reveal serpentine channels 74 and 76, respectively, which
pneumatically couple the various input and output ports of the amplifier
stage relay 10 from a pressure inlet 78 and the pressure outlet 17 to
provide alternate modes of operation. As illustrated in FIG. 1, the first
port switch 70 is positioned such that the input port 13 is in
communication with the pressure inlet port 78, and the input port 11 is
vented to atmosphere. The second port switch 72 is shown to vent the
output port 14. It should be appreciated from U.S. Pat. No. 4,974,625
that this switch configuration places the amplifier relay stage 10 in a
reverse/snap-acting mode, which when combined with the reverse-acting
signal stage valve 110 provides a direct/snap-acting pneumatic control
device 1.

[0012]That is, a decrease in pressure in a chamber 88 results in movement
of a cage assembly 59 to the left with respect to FIG. 2, which provides
an increasing output pressure at the pressure outlet 17. Thus, in
operation when an increasing input signal moves the stem tip 135 of the
signal stage valve 110, the reverse-acting mode of the signal stage valve
110 provides a decrease in its output pressure in passageway 82 and
consequently a decrease in pressure in the chamber 88 to provide a
direct-acting pneumatic control device 1. The alternate switch
configuration for the control device 1 couples the input port 11 to the
pressure inlet port 78 and the input port 13 is vented to atmosphere with
the second port switch 72 configured to couple port 14 to the output port
12. This alternate configuration places the amplifier stage relay 10 in a
direct/snap-acting mode and, therefore, the pneumatic control device 1
operates in a reverse/snap-acting mode. The remaining possible switch
configurations for the amplifier stage relay 10 render the relay
inoperable because there is no feedback mechanism present in the
described embodiment of control device 1.

[0013]As shown in FIG. 2, the signal stage valve 110 includes a single
plug 130, a first valve seat 120 and a second valve seat 122. In a first
state, a first plug end 132 does not engage the first valve seat 120 and
a second plug end 134 engages the second valve seat 122. In a second
state, the first plug end 132 engages the first valve seat 120 and the
second plug end 134 does not engage the second valve seat 122. In an
intermediate state, neither plug end 132 and 134 engages either of the
respective valve seats 120 and 122.

[0014]In operation, a linkage may apply a force to the valve stem tip 135
to move it toward the amplifier relay 10 or to the right (with reference
to FIG. 1 and FIG. 2). The rightward movement of the valve stem tip 135
causes movement of the stem 130 of the signal stage valve 110 that
results in the first plug end 132 and the second plug end 134 being
simultaneously separated from their respective first and second valve
seats 120 and 122 in the intermediate state. During this separation, the
supply gas, such as natural gas, from a supply port 85 is vented or bled
through the second valve seat 122 to the atmosphere past valve stem tip
135. This venting to atmosphere of the supply gas is often called
transition bleed, which may cause excessive loss of supply gas, such as
natural gas, to the atmosphere. When the rightward movement of the stem
130 continues, the stem 130 ultimately engages the first plug end 132
with the first valve seat 120 and the transition bleed ceases, and the
fluid pressure within a through feedback passage 114 of the signal stage
valve 110 and the chamber 88 of the amplifier stage relay 10 is at
atmospheric pressure.

[0015]The change from supply gas pressure to atmospheric pressure within
the chamber 88 results in the diaphragm cage assembly 59 being moved
toward the left in FIG. 2 by a spring 48 in the chamber 16. The cage
assembly 59 includes a valve seat 30 and valve plug 40. The leftward
movement of the valve seat 30 and the valve plug 40 causes a valve plug
38 to engage a valve seat 42 and terminate the transmission of supply gas
to the output port 12. The valve seat 30 is then moved away from the
valve plug 40 as the diaphragm cage assembly 59 moves to the left so that
fluid pressure in the chamber 16 flows through the T-shaped opening to
the chamber 18 to vent the fluid pressures from the chambers 16 and 18.

[0016]While the use of the signal stage valve 110 with the amplifier stage
relay 10 provides sensitivity to the input signal from the linkage, it
also provides a significant transition bleed of natural gas during the
operation of the dual-stage pneumatic control device 1. It should also be
appreciated that one way to reduce the transition bleed and maintain most
of the gain of the dual-stage pneumatic control device 1 is to couple
together two amplifier stage relays 10 for serial operation. However,
coupling the two amplifier stage relays 10 together to create a tandem
device increases the cost and results in a relatively larger, dual-stage
pneumatic control device 1.

[0017]In addition, while certain designs may provide a feedback force to
the above-described device, it may be less desirable. One approach is to
provide a diaphragm between the stem 130 and the valve body 112 in the
signal stage valve 110. However, the diaphragm has to be clamped or
retained at its inner and outer diameters, which results in a larger
signal stage that subsequently requires undesirable changes in the
linkage and the displacer.

SUMMARY

[0018]An example fluid flow control apparatus described herein includes a
signal stage comprising a signal stage relay having a supply plug being
operatively connected to a valve seat at a first end and an exhaust seat
at a second end and a seal operatively coupled to the supply plug such
that the seal provides a feedback area to apply a fluid pressure feedback
force to the exhaust seat.

[0019]In yet another example, a dual-stage fluid flow control apparatus
described herein includes a signal stage having a proportional output,
the signal stage comprising a signal stage relay including a supply plug
having a first end adjacent a valve seat and a second end adjacent an
exhaust seat, a signal stage input post is adapted to couple the signal
stage to a control device, and means for urging a seat load across the
supply plug toward either the valve seat or the exhaust seat. An
amplifier stage comprising an amplifier stage relay is operatively
connected to the signal stage via a signal passage, the amplifier stage
having a fluid supply responsive member adapted to move a relay member to
provide an amplified fluid supply output such that a shift in the seat
load across the valve seat and the exhaust seat provides a predetermined
engagement of either the valve seat to the first end of the supply plug
or the exhaust seat to the second end of the supply plug to provide
either a proportional or snap-acting and a direct or reverse acting
output of the amplifier stage relative to an input signal at the signal
stage input post.

[0020]In yet another example, a fluid flow control apparatus described
herein includes a signal stage having a proportional output. The signal
stage comprises a signal stage relay including a supply port, a supply
plug having a first end adjacent a valve seat and a second end adjacent
an exhaust seat, a signal stage input post adapted to couple the signal
stage to a control device and means for urging a seat load across the
supply plug toward either the valve seat or the exhaust seat. An
amplifier stage comprising an amplifier stage relay is operatively
connected to the signal stage via a signal passage. The amplifier stage
relay having a fluid supply responsive member adapted to move a relay
member to provide an amplified fluid supply output such that a shift in
the seat load across supply plug of the signal stage closes the exhaust
seat of the signal stage prior to opening the valve seat of the signal
stage to substantially eliminate a transition bleed in the signal stage.

BRIEF DESCRIPTION OF THE DRAWINGS

[0021]FIG. 1 is a cut-away illustration of port switches of the amplifier
stage of the dual-stage pneumatic control device of FIG. 2.

[0022]FIG. 2 is a cut-away illustration of a known dual-stage,
direct-acting pneumatic control device.

[0023]FIG. 3 is a cut-away illustration of an example dual-stage,
direct-acting pneumatic control device at a quiescent operating point.

[0024]FIG. 4 is a cut-away illustration of an example dual-stage pneumatic
control device with stabilizing pressure regulators.

[0026]In general, the example apparatus and methods described herein may
be utilized for controlling fluid flow in various types of fluid flow
processes. An example fluid flow control apparatus includes a dual-stage
fluid control device having a compact, low bleed signal stage with
proportional output to improve the control of fluid flow. Additionally,
while the examples described herein are described in connection with the
control of product flow for the industrial processing industry, the
examples described herein may be more generally applicable to a variety
of process control operations for different purposes.

[0027]FIG. 3 is a cut-away illustration of an example direct-acting
dual-stage pneumatic control device 200 comprising a signal stage having
signal stage relay 300 and an amplifier stage further comprising an
amplifier stage relay 210. The direct-acting signal stage relay 300
provides a signal stage C and the direct-acting amplifier stage relay 210
provides an amplifier stage D of the example dual-stage pneumatic control
device 200. The amplifier stage relay 210 of the amplifier stage D is
similar to the four-mode pneumatic relay valve disclosed in the U.S. Pat.
No. 4,974,625 and the amplifier stage relay 10 disclosed in FIG. 2,
including the port switches 70 and 72 illustrated in FIG. 1. Those
components in the amplifier stage relay 210 of FIG. 3 that are the same
as or similar to the components in the amplifier stage relay 10 of FIG. 2
have the same reference numerals increased by 200.

[0028]As described in detail below, it should be appreciated by those of
ordinary skill in the art that the signal stage relay 300 improves the
operation of the previously described dual-stage relay illustrated in
FIG. 1 and FIG. 2 by providing a throttling or proportioning action,
thereby permitting utilization of the four modes available in the
amplifier stage relay 210 while substantially reducing the transition
bleed associated with signal stage valve 110. A throttling or
proportional/direct mode of operation is described below as an example
operation of the control device 200. Those desiring more detail or
description should refer to U.S. Pat. No. 4,974,625, which describes
therein the other three modes of operation of a four-mode pneumatic relay
valve similar to the amplifier 210 of FIG. 3.

[0029]Referring to FIG. 3, the signal stage relay 300 of the signal stage
C includes a relay body 312 having a through feedback passage 314, a
transverse port 316, an inlet 318, a first valve seat 320, and a second
valve seat 322. The second valve seat 322 is located on an exhaust seat
325 having a seal or an o-ring 326 engaging and sealing against an inner
surface 317 of the through feedback passage 314. As described in greater
detail below, the o-ring 326 provides an effective area on which fluid
pressure in the feedback passage 314 of the signal stage relay 300 may
act to create a feedback force to provide the throttling or proportioning
action in the control device 200.

[0030]It should be appreciated that at a quiescent point in the throttling
or proportional mode, valve plugs 330, 240 and 238 are in a "closed"
position. That is, closed position means the valve is "substantially in
contact with" the valve seat. However, one skilled in the art appreciates
that for such a valve seating surface, for example, a metal-to-metal
valve seat arrangement, in a closed position with the limited seat loads
available such valve-seat arrangements are known to leak small quantities
of fluid (i.e. not bubble tight). This leakage at the seats yields a
fluid flow to provide throttling action of the pneumatic control device
in operation. That is, unlike a snap-acting operation wherein the valves
are substantially moving into and out of contact with the valve seats, a
throttling or proportional mode is, in part, defined by shifts in
corresponding seat load to modify a pressure balance across the relay
components. The shifting seat loads provide a modification in seat
leakage during quiescent operation to shift the pressure balance across
the signal stage C and the amplifier stage D in proportion to supply
input and sensor feedback. It should also be appreciated that other
materials of construction having sufficient hardness will yield similar
leakage flows during operation.

[0031]As shown in FIG. 3, the exhaust seat 325 has an input post 327 and
is retained within the through feedback passage 314 by an end cap 329.
The supply valve plug 330 is located in the through feedback passage 314
and includes a first plug end 332 adjacent (e.g., situated immediately
adjacent) the first valve seat 320 and a second plug end 334 adjacent
(e.g., situated immediately adjacent) the second valve seat 322. The
exhaust seat 325 includes a shoulder 336 that receives a spring 340. The
spring 340 also engages a shoulder 313 to urge the exhaust seat 325 into
engagement with the end cap 329 and away from the second plug end 334. A
second spring 344 engages a valve body shoulder 315 and the first plug
end 332 to urge the first plug end 332 into engagement with the first
valve seat 320.

[0032]The signal stage relay 300 is positioned within an opening 280 in an
end cover 236 of the amplifier stage relay 210. An end cover 236 includes
a signal passage 282 that fluidly couples the transverse port 316 of the
signal stage relay 300 with a signal chamber 288 defined partially by a
signal diaphragm 290 located between the end cover 236 and an
intermediate piece 239. The end cover 236 also includes a supply port 285
that provides supply gas to the inlet 318 of the signal stage relay 300.

[0033]In a quiescent operational mode, the first plug end 332 is in
contact with the first valve seat 320 and the second plug end 334 is in
contact with the second valve seat 322. A supply gas is provided to the
signal stage relay 300 via the supply port 285 and the inlet 318. The
first plug 332 is seated at the first valve seat 320 with sufficient seat
load so that the supply gas is substantially prohibited from passing the
first valve seat 320 and the seat load of the second plug end 334 is
seated at the second valve seat 322 of the exhaust seat 325 so supply gas
is substantially prohibited from exhausting from the exhaust seat 325.
However, as previously explained, in throttling or proportional mode, at
a quiescent operating point, both first and second valve plug ends 332
and 334, when engaged with the respective valve seats 320 and 322,
substantially prohibit fluid flow, with only a leakage flow present. The
slight leakage creates a proportional, shifting pressure balance across
the signal and amplifier stages C and D to modify the respective seat
loads in proportion to the supply fluid wherein a feedback force is
coupled through a linkage connected to a displacer in a fluid tank (not
shown). The input signal may be derived from any number of well-known
inputs including pressure signals and direct mechanical forces.

[0034]For example, the supply plug 330 is shown in its left most position,
with respect to FIG. 3, in contact with the first valve seat 320. In
operation, such as a level control application, a buoyant force is
applied to a displacer by a fluid in the fluid tank, an input or
mechanical linkage provides an input force to the input post 327 of the
exhaust seat 325. The input force or signal increases the leakage flow
across the first valve seat 320. This action also causes the seat load of
the second valve seat 322 to sealingly engage the second plug end 334 and
decrease leakage flow through feedback passage 314 to the atmosphere, and
then the first plug end 332 to increase leakage flow from the first valve
seat 320 to enable a limited quantity of supply gas to enter the feedback
passage 314.

[0035]Subsequently, the supply gas from the supply port 285 passes through
the inlet 318, the first valve seat 320, through the feedback passage 314
to the transverse port 316, the signal passage 282 and the signal chamber
288 to act upon the signal diaphragm 290. The pressure of the supply gas
increases a force supplied by the signal diaphragm 290 and a diaphragm
cage assembly 259, thereby increasing a seat load upon a valve seat 230
from the valve plug 240 to decrease a leakage flow therebetween. This
pressure also acts upon the inner surface 317 of the o-ring 326 to apply
a negative feedback force on the linkage to provide a proportional output
from the control device 200. That is, a force equal to the product of the
pressure within the signal passage 282 and the effective sealing area of
the o-ring 326 (i.e. the cross-sectional area of the o-ring defined by
the inner surface 317) is applied in opposition to the linkage force.

[0036]As the linkage applies the input signal to the input post 327
seating forces between the first plug end 332 and the first valve seat
320 are diminished or reduced, increasing supply gas pressure to the
signal chamber 288. The amplifier stage relay 200 of the amplifier stage
D has port switches (not shown) set for proportional/direct operation.
Thus, supply gas is applied to an input port 211 and a chamber 215. A
chamber 216 and an output port 217 are coupled to a final control device.
The supply gas is contained within the chamber 215 as long as a leakage
flow across a valve seat 242 is substantially reduced by the valve plug
238 to prohibit a pressure increase in the chamber 216 and the output
port 217. As pressure increases in the signal chamber 288, the force
generated by the signal diaphragm 290 and the diaphragm cage assembly 259
increases the seat load across the valve seat 230. As the seat load
increases across the valve seat 230 and the valve plug 240 of a plug
assembly 237, the seat load across the valve seat 242 and the plug 238
decreases. The decrease in seat load across the valve seat 242 and the
plug 238 increases a leakage flow from the chamber 215 and subsequently
into the chamber 216. The increase in flow and pressure communicate
through the pressure outlet 217 and into the final control device.

[0037]Continuing in operation, as the seat load of the first plug end 332
and the first valve seat 320 decreases, the supply gas in the feedback
passage 314 acts upon the exhaust seat 325 to offset the input signal
applied to the input post 327 by the linkage and provide a proportional
amount of supply gas pressure to the signal chamber 288. At equilibrium,
the valve seat 230 of the amplifier stage relay 210 is in contact with
the valve plug 240 and the valve seat 242 is in contact with the valve
plug 238 with the seat loads in balance so that the output pressure at
the pressure outlet 217 and the final control device is proportional to
the input signal at the input post 327.

[0038]If the input signal at the input post 327 decreases, the force
provided by the diaphragm cage assembly 259 decreases so that the seat
load between the valve plug 238 and the valve seat 242 increases and the
seat load between the valve seat 230 and the valve plug 240 decreases. In
this state, the leakage flow between the valve seat 230 and the valve
plug 240 enable the supply gas in the chamber 216 to pass through a
T-shaped opening 232 to the chamber 218 and vent through an input port
213, which is exposed to the atmosphere. Changes in the input signal at
the input post 327 results in a new equilibrium state for the amplifier
stage relay 210 with the output pressure at the pressure outlet 217 being
directly proportional to the input signal.

[0039]During operation, when the input force at the input post 327
decreases, the seat load at the second valve seat 322 decreases and the
supply plug 330 is slightly loaded. That is, the seat load at the first
plug end 332 of the supply plug 330 and the first valve seat 320
increases to decrease the leakage flow of supply gas through the first
valve seat 320. The seat load at the second valve seat 322 of the exhaust
seat 325 and the second plug end 334 of the supply plug 330 decreases.
The decrease in seat load permits the supply gas in the signal chamber
288, the signal passage 282, the transverse port 316, and feedback
passage 314 to vent through the second valve seat 322 to atmosphere.

[0040]The signal stage relay 300 enables the example dual-stage pneumatic
control device 200 to have a high gain, a low transition bleed, and four
modes of operation that achieve numerous advantages. For example, the
spring 340 is utilized to overcome a frictional force created by the seal
or O-ring 326 and to keep or maintain the input post 327 in contact with
the input linkage, thereby ensuring that a dead band of operation does
not occur during the operation of the linkage. In other words, the input
post 327 is in contact with the input linkage such that a bias force of
the spring 340 substantially maintains contact between the input linkage
and the input post 327 to substantially eliminate a dead band between the
input linkage motion and exhaust seat 325 motion. The high gain,
four-modes of operation provided by the example dual-stage pneumatic
control device 200 eliminate the need to use either two-serially aligned
amplifier stage relays 210 to provide a high gain or a diaphragm between
the exhaust seat 325 and the valve body 312 to provide a feedback force.
The use of the seal or O-ring 326 (i.e., as opposed to the use of a
diaphragm) to provide a supply gas pressure feedback force to the exhaust
seat 325 enables the signal stage relay 300 to have a small diameter and,
thus, a small and compact size. This also results in the example
dual-stage pneumatic control device 200 being usable with a smaller
displacer and lighter fluids in a fluid vessel, thereby minimizing the
cost of the fluid vessel.

[0041]The example dual-stage pneumatic control device 200 utilizes the
springs 244 and 248 of the amplifier stage relay 210 and the springs 344
and 340 of the signal stage relay 300 to assist in the control of the
flow of the supply gas through or across the respective valve seats 242,
230 and 320 and 322. As a result, the example dual-stage pneumatic
control device 200 may function at any orientation, including horizontal,
vertical, and angled without compensating for the affects of gravity.

[0042]One skilled in the art should also appreciate that the feedback
area, presented by the effective area of the o-ring 326 can also be
adjusted by changing the internal diameter of the feedback passage 314 of
the signal stage relay 300 and the external diameter of the seal or
o-ring 326. That is, the signal stage relay housing 312 and the seal or
o-ring 326 can be quickly changed or replaced as a replaceable single
stage module that provides a predetermined feedback area to accommodate
different types of services such as water, condensate or interface, which
may provide or exert different linkage forces. For example, a relatively
large feedback area (e.g. 0.1080 in2) would be preferable for
applications providing a large buoyant force (i.e. corresponding to fluid
having an approximate specific gravity of 1.0), such as water. A slightly
smaller feedback area (e.g. 0.0625 in2) would accommodate
applications providing a moderate buoyant force (i.e. corresponding to a
fluid having an approximate specific gravity of 0.8) such as oil and a
very small feedback area (e.g. 0.036 in2) would preferably
accommodate an oil-to-water interface application with a small buoyant
force (i.e. corresponding to fluids having an approximate differential
specific gravity of 0.1). Specifically, one of ordinary skill in the art
will recognize that this feature provides the user with an improved setup
and calibration scenario for level control applications since the lever
and the displacer need not be modified or replaced for these different
applications.

[0043]The example dual-stage pneumatic control device 200 depicted in FIG.
3 may provide very high gain (i.e. increased responsiveness) and very low
gas consumption during normal operation. However, in certain applications
such high gain or responsiveness may create susceptibility to mechanical
vibrations that may lead to instability in control. The source of this
instability is generally the rapid application of a feedback force on a
controller linkage by the signal stage of the pneumatic controller
device. The example pneumatic control device 401 of FIG. 4 may
substantially reduce such susceptibility by: 1) independently controlling
the pressure to the signal stage relay; and 2) reducing the feedback area
of signal stage relay.

[0044]Referring to FIG. 4, a cut-away illustration of an example
dual-stage pneumatic control device 401 having a signal stage E and an
amplifier stage F including stabilizing pressure regulators 500 and 510.
The stabilizing pressure regulators 500 and 510 independently provide
supply air to a signal stage relay 410 and an amplifier stage relay 420
through a signal supply pressure inlet 485 and an amplifier supply
pressure inlet 411. It should be appreciated that such stabilizing
pressure regulators 500 and 510 could be integrated within the signal
stage E and the amplifier stage F, or such regulators could be external
to the signal and amplifier stages E and F. Alternatively, it should be
appreciated that stabilizing regulator 500 may be positioned downstream
of stabilizing pressure regulator 510. The signal stage relay 410 and the
amplifier stage relay 420 of example device generally function as the
previously described example dual-stage pneumatic control device 200
depicted in FIG. 3 except the stabilizing pressure regulators 500 and 510
provide independent pressure supply to each stage, signal stage E and
amplifier stage F to enhance device stability and to improve overall
pneumatic control device performance. For example, the signal stage
pressure regulator 500 may be set to 8 psig, whereas the amplifier stage
pressure regulator 510 may be set to 35 psig. Generally, the signal stage
E is set to a lower pressure than the amplifier stage F. That is, the
signal stage pressure may be set at a minimal operating point to operate
the amplifier stage F. The lower signal stage pressure improves pneumatic
control device stability and performance in the following manner: 1)
lower signal stage supply pressure directly reduces the feedback force
that can be generated by the signal stage relay 410 (i.e.
Force=Pressure×Area); and 2) lower pressure directly reduces the
gas consumed by the signal stage relay 410.

[0045]Additionally, FIG. 5 illustrates a signal stage 610 to further
improve pneumatic control device performance. That is, in combination
with the low signal stage pressure of the example pneumatic control
device of FIG. 4, the present example signal stage 610 has a reduced
feedback area to further reduce feedback forces on a sensor. The example
signal stage relay 610 includes a relay body 612 having smaller internal
diameter relative to the feedback passage 614 and/or the previously
described relay body 312 of the example pneumatic control device 200
depicted in FIG. 3. The corresponding feedback passage 614 is also
reduced in diameter to provide a sealing engagement with a seal or an
o-ring 626. As previously described, the fluid pressure in the feedback
passage 614 acts upon the inner surface 617 and the seal or o-ring 626 to
apply a negative feedback force on the linkage to provide a proportional
output from a control device. As a result, the reduced feedback area
provides a reduced feedback force to a sensor coupled to a pneumatic
control device.

[0046]The combination of low pressure signal stage and the reduced
feedback-area signal stage may improve device stability for feedback
sensors with high gain. By controlling the feedback area in a
predetermined manner and configuring signal stage pressure independent of
amplifier stage pressure, a pneumatic control device can be adapted to
stabilize a broad variety of displacement-style level controllers.

[0047]In summary it should be appreciated that the example device
disclosed herein substantially eliminates the transition bleed of the
control device fashioning a dual-stage pneumatic relay that positively
closes an exhaust port of the relay before a supply port opens.
Additionally, a seal or an o-ring of a signal stage relay provides
significant negative feedback area to counteract or offset the lever
force on the signal stage relay in a throttling or proportioning manner
while providing increased gain to improve overall system performance.

[0048]Although certain example apparatus have been described herein, the
scope of coverage of this patent is not limited thereto. On the contrary,
this patent covers all methods, apparatus and articles of manufacture
fairly falling within the scope of the appended claims either literally
or under the doctrine of equivalents.